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Pests and Diseases
Algal leaf spot (American Samoa) - Photos and report submitted by Dr. Fred Brooks, Plant Pathologist


algal leaf spot
The leaf spot lesions are a light brown color at first, usually surrounded by a noticeable and diffuse yellow halo.  In wet weather they are water-soaked; the centers usually fall out of mature lesions.


algal leaf spot
Close-up of algal leaf spot, showing brown colored lesion surrounded by a diffuse, yellow halo.



algal thallus
Close-up image of the algal thallus on the surface of an infected noni leaf.

Pathogen: The disease is caused by a plant-parasitic algae, Cephaleuros minimus. Cephaleuros species are the only plant parasitic algae known.

Symptoms: Leaf spots (approx. 1-2 cm diameter), characterized by a light brown color and surrounded by a conspicuous, diffuse, yellow halos. The pathogen, Cephaleuros minimus, enters the leaf though the upper surface and forms a filamentous thallus above the epidermis which moves intercellularly to the lower leaf surface.  Sporangia and setae break through the lower epidermis and can usually be seen with a hand lens due to their bright orange color. The effect of these symptoms may be minor; the disase is not fatal but some premature defoliation may occur.

Disease distribution: As of 2005, the disease has only been reported from American Samoa. It is reported to be the most common leaf spot disease of noni in American Samoa.

Epidemiology: Algal leaf spot is favored by very wet, warm and humid weather and is most common where noni is growing in low-light conditions as an understory plant that is shaded by other species. Spores are dispersed by primarily splashing rain and by wind and wind-driven rain.
  1. Sanitation (removal of severely diseased leaves from the plant; removal and destruction of fallen infected leaves)
  2. Moisture and humidity management (ensure good drainage, control weeds, adequate plant spacing, pruning, minimize leaf wetness and overhead irrigation)
  3. Grow noni in full sun.
Notes: This is probably the first report of this disease in the Pacific and worldwide, although it is unlikely that the disease is actually confined to American Samoa.

Last Updated on December 7, 2006